Syrian activists inspect the bodies of people they say were killed by nerve gas in the Ghouta region, in the Duma neighbourhood of Damascus, on 21 August, 2013. Photograph: Reuters

In the high-stakes drama over chemical weapons use in Syria, the US, France and Britain have all made clear that they hold the Assad regime responsible for the Ghouta attack on August 21st. Syria rejects the charge, and like its close ally Russia, blames the rebels. Neither have yet produced any evidence to support their position. Germany's intelligence assessment adds some intriguing new detail.

According to Der Spiegel, Gerhard Schindler, the head of the BND external intelligence service, told MPs in Berlin on Monday that while there was still no "incontestable proof," analysis of the evidence has led his service to believe that Assad's regime is to blame. Schindler also emphasized that the rebels were unable to carry out such a concerted attack.

In line with the three other western assessments, the German spy chief stressed the size of Syria's CW arsenal and its ability to use it. Schindler also believes CW had been used on a smaller scale before August 21. Britain's Joint Intelligence Committee assessment counted 14 separate incidents — though it has not publicized its evidence. Schindler said that in the earlier attacks the poison gas mixture was diluted, explaining the much lower death tolls in those assaults.

The UK assessment, the sketchiest of the three published reports, admitted that it could not explain Assad's motivation, especially given the presence in Damascus of UN inspectors investigating previous incidents. But Germany has followed France and the US in suggesting that chemical weapons had been used to intimidate the rebels and capture territory in a crucial battle for Damascus, especially to the east of the capital.

There is a twist: "It could also be the case that errors were made in mixing the gas and it was much more potent than anticipated," Schindler said. Estimates of fatalities range from the US figure of 1429 to the French one of 281.

Schindler also presented an additional clue, one that has not thus far been made public. He said that the BND listened in on a conversation between a high-ranking member of the Lebanese militia Hezbollah, which supports Assad and provides his regime with military assistance, and the Iranian embassy (it does not say whether this was in Beirut or Damascus) . The Hezbollah functionary, Schindler reported, seems to have admitted that poison gas was used. He said that Assad lost his nerve and made a big mistake by ordering the chemical weapons attack.

The new information from the BND could become important in the coming days. Thus far the US has only noted that after the attack, intelligence agencies had intercepted internal government communications indicating concern about a possible UN inspection of the site. The telephone conversation intercepted by the BND could be an important piece in the puzzle currently being assembled by Western intelligence experts.

The German take on the current crisis is likely to be taken seriously. The BND has a track record of good intelligence "coverage" of Lebanon and Syria (and Iran) and has often played a role mediating between Israel, Damascus and Hezbollah.